The invention relates to a method of controlling a drive system and/or a wheel brake system.
In the case of motor vehicles having driver assistance systems, by which particularly a predefined speed or a predefined distance from a vehicle driving ahead is to be kept constant, methods are known by which a drive system and/or a wheel braking system can be controlled for achieving the driver assistance goals. Problems occur here again and again, particularly during downhill driving. Either the wheel braking systems are overheated, or operations of the drive system, such as the downshifting of automatic transmissions, or the change of coasting operation types (fired/non-fired), result in losses of comfort.
Examples of known methods of controlling drive systems and wheel braking systems, which generally already provide an increase of the drive braking torque or of the “engine brake” for protecting the wheel brake, are the objects of German Patent Documents DE 102 51 563 A1, DE 43 29 918 A1 and DE 197 07 140 C1.
It is an object of the invention to improve a method of controlling a drive system and a wheel braking system with regard to relieving the wheel braking system while comfort is simultaneously taken into account.
In the case of the method of the invention for controlling a drive system and a wheel braking system, a total braking torque consisting of a drive braking torque and a wheel braking torque to be applied to the wheels of a motor vehicle is particularly advantageously adjusted such that the desired wheel braking torque is reduced by an amount which can be compensated by a (preferably maximally) possible increase of the drive braking torque when the wheel braking system is particularly loaded (particularly when driving downhill) and when the vehicle velocity is approximately constant (steady-state operating mode). However, the increase of the drive braking torque is limited to a predefined amount (achievable, for example, in the fired coasting operation) when the wheel braking system is less loaded or the vehicle velocity changes (dynamic operating mode).
The total braking torque may, for example, be predefined by the driver's wish determined by means of a braking and/or accelerator pedal interpretation or by a driver assistance system (“cruise control”, “ACC”, etc.). The invention is particularly advantageous when it is used in motor vehicles with driver assistance systems aiming at keeping a predefined speed constant.
The presence of a defined increased loading of the wheel braking system can be recognized when, for example, a predefined wheel braking torque threshold is exceeded for a predefined minimal duration. However, operating parameters, such as temperatures pertaining to the wheel braking system or brake pressure values and the rotational speed or the velocity, can also be used for estimating the loading of the wheel braking system. Navigation data, by which a downhill drive can be anticipated, may even be used for estimating a loading of the wheel braking system to be expected.
The wheel braking torque is the fraction of the total braking torque applied to the wheels which is applied by the wheel braking system.
The drive braking torque is preferably the fraction of the total braking torque applied to the wheels which is applied by the entire drive system consisting of at least an engine and a transmission, in which case the engine as well as the transmission may be electronically controllable.
The reduction of the desired wheel braking torque first indicates a mathematically possible reduction. For, when, in the case of a not-by-wire braking system, the driver operates the brake pedal, an actual reduction of the wheel braking torque carried out by the wheel braking system can possibly not (immediately) be carried out. However, in this case, the drive braking torque according to the invention is nevertheless increased, so that first an addition of the increased drive braking torque to the total braking torque is carried out. However, it is expected that subsequently, because of the resulting noticeable braking effect, the driver will reduce the wheel braking pressure by returning the brake pedal. The driver is therefore used as part of the method according to the invention with respect to the actual reduction of the wheel braking pressure. In the case of an automatic wheel braking pressure control without operating the brake pedal or, in the case of an automatic wheel braking pressure control during interventions of a drive assistant system, particularly a cruise control system, the reduction of the desired wheel braking torque can be carried out immediately and fully automatically.
The invention is based on the following recognitions:
The first goal is the reduction of the braking load, in which case the exceeding of the braking load also comprises the exceeding of the braking load to be expected, so that the invention also intervenes preventively before the wheel braking system is actually considerably loaded. Basically, while taking into account demands on the drive system for relieving the wheel braking system, a drive braking torque which is as high as possible should be set. Here, the increase of the drive braking torque may first have an arbitrary course and may preferably always be adjusted in a maximally possible manner. The second goal is also the increase of comfort by taking into account driving conditions under which jumps in wheel torques are particularly noticeable to the driver. Tests have shown that changes, particularly sudden changes, as during a change from a fired coasting operation to a non-fired coasting operation and vice-versa, in the course of the increase of the drive braking torque, may have a particularly disadvantageous effect with respect to comfort in a dynamic operating mode. A dynamic operating mode manifests itself particularly in a change of the vehicle velocity. An increase of the drive braking torque without any limitation is therefore preferably permitted only at an at least almost constant vehicle velocity.
An embodiment of the invention is illustrated in the drawing. It applies to the case in which the total braking torque is predefined as a desired value by a driver assistance system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.